1. Field of the Invention
This invention relates, in general, to electrical inductive apparatus and, more specifically, to single-phase power transformers.
2. Description of the Prior Art
Electrical inductive apparatus, such as large power transformers of the shell-form type, are conventionally constructed with two core loops placed side-by-side, with the electrical windings encircling the adjacent portions of the two core loops. A more effective utilization of core material may be obtained by using two winding assemblies disposed on a single core loop. However, when this construction is applied to high KVA ratings, severe heating of the tank walls is experienced adjacent the portions of the electrical windings not magnetically shielded by the magnetic core.
Several arrangements have been developed to reduce the heating in the tank walls which do not rely upon the placement of magnetic shielding laminations or bundles along the tank wall. U.S. Pat. No. 3,967,226, which is assigned to the assignee of this invention, discloses an arrangement wherein shielding magnetic core loops are located on each side of the main magnetic core of a single-phase shell-form transformer. The rectangular main magnetic core includes two concentric portions or loops which are spaced apart for cooling of the inner core laminations. Two shielding magnetic cores are positioned adjacent opposite ends of the main magnetic core and serve to direct much of the leakage flux away from the tank walls. Although such a structure provides adequate cooling of the core laminations and shielding of the tank walls, the support structure required to support the magnetic cores from the transformer tank experiences a non-uniform weight distribution due to its non-symmetrical configuration. The support structure includes a beam having a flange and two parallel web portions, with the web portions respectively extending into the gaps between adjacent legs of the main magnetic core and the outer magnetic shielding core loops. The length of one of the web portions of the support structure must be smaller than the length of the outer web portion to fit in the gap between the leg of the inner and outer portions of the main magnetic core. In addition, the web portions are not centered on the flange due to the difference in the widths of the main and shielding magnetic core loops which results in a non-uniform beam structure.
Thus, it would be desirable to provide a magnetic core structure having outer shielding core loops which has a symmetrical support structure for uniform weight distribution.
In recent years, there has developed a need in the electrical industry for single-phase transformers having higher and higher KVA ratings. Conventional magnetic cores, as described above, constructed to handle to higher KVA ratings may exceed the maximum rail shipping width clearance dimensions. It is known that the capacity or rating of a single-phase transformer may be increased without changing its width or height dimensions by adding additional core loops in line with the main magnetic core as shown in U.S. Pat. No. 1,765,483, which is assigned to the assignee of this invention. U.S. Pat. No. 3,156,886 discloses a similar arrangement in which the magnetic core includes at least six legs which are conected by common yoke members. However, the magnetic core shown therein is of the core-form type which differs significantly from shell-form magnetic cores since the winding legs of the magnetic core extend in the vertical direction instead of horizontal and thereby require a different support structure if a split-type core construction is to be utilized. In addition, at the higher ratings desired for single phase transformers, magnetic cores constructed by either of these arrangements may still exceed the maximum rail shipping width dimension.
Furthermore, it is impossible to add multiple core loops to the magnetic core structure shown in U.S. Pat. No. 3,967,226 due to the presence of the smaller shielding core loops and the requirement that all the magnetic core loops be of equal cross section, at the higher ratings, for uniform flux distribution.
Thus, it is desirable to provide a shell-form type power transformer wherein higher KVA ratings may be obtained without exceeding the maximum rail shipping width clearance dimensions. It is also desirable to provide a single-phase shell-form type power transformer wherein the magnetic core is constructed to provide adequate cooling of the internal core laminations. Finally, it is desirable to provide a shell-form type power transformer which utilizes outer magnetic core loop to direct the leakage flux away from the adjacent wall surfaces of the tank transformer.